1. Field of the Invention
The subject application generally relates to a planar antenna, and more particularly, to a planar antenna of a handheld device.
2. Description of Related Art
Multi-input multi-output (MIMO) is a term used for describing the transmission of radio signals between multiple antennas. In short, MIMO refers to the use of multiple antennas respectively at a transmitter and a receiver, wherein signals are transmitted and received by the antennas at the transmitter and the receiver so that the service quality provided to each user is improved. Compared to a conventional signal-antenna system, the MIMO technology offers an increased frequency available ratio such that the system can transmit data more efficiently with limited wireless bandwidth.
FIG. 1 is a diagram of a conventional MIMO handheld device. FIG. 2 is a diagram illustrating the signal quality (VSWR) of a planar antenna in FIG. 1. Referring to FIG. 1 and FIG. 2, a handheld device 100 adopts two planar antennas 110 and 120. The planar antenna 110 has a feed point F110 and a ground point G110. The planar antenna 120 has a feed point F120 and a ground point G120. Because the planar antennas 110 and 120 have similar operating frequencies, signals transmitted and received by the planar antennas 110 and 120 may interfere with each other. The interference cannot be effectively eliminated (as shown in FIG. 2) even when the planar antennas 110 and 120 are respectively disposed at two different sides of the handheld device 100. In FIG. 2, the curve 131 indicates the transceiving quality of the planar antenna 110, the curve 132 indicates the transceiving quality of the planar antenna 120, and the curve 133 indicates the situation of signal interference.
Generally speaking, a planar antenna requires a clearance area. If two planar antennas are respectively disposed at two different sides of a handheld device, a greater total clearance area is required by the two planar antennas and which is disadvantageous to the circuit layout of the handheld device. Besides, there may not be enough space for respectively disposing two planar antennas at two different sides of a handheld device. The closer the two planar antennas are, the more serious the problem of signal interference is. Moreover, the problem of signal interference is aggravated if three or more antennas are disposed in a handheld device.